The present invention relates to data delivery on telecommunications links and, in particular, to an improved apparatus and method for power control data delivery for digital subscriber access to data services through the Public Switched Telephone Network (PSTN) using wire loops.
An emerging trend in the telecommunications industry is to provide data services deployed on existing telephone twisted pair copper wires (local loops) utilizing a frequency spectrum above the voice frequency band. These services are susceptible to interference created by frequency compatible and frequency incompatible services carried in the same and/or adjacent binder groups. A binder group is a bundle of twisted pairs of copper wires bound together in a cable consisting of 12, 25, 50 or 100 such twisted pairs. Each twisted pair provides voice and/or data services to a subscriber. As high frequency services are added to a binder group, the high frequency traffic causes interference called xe2x80x9ccross-talkxe2x80x9d that reduces the effective loop reach and transmit capacity of data services in the cable. Consequently, loop reach and data capacity of data services using frequencies above the voice frequency band suffer from a loss in link reliability as more data service subscribers are added to a binder group.
Access to the data services is generally provided by a collection of provider transceivers at a central site connected through twisted pairs to subscriber transceivers. The twisted pairs are of various lengths due to the different location of each subscriber and the physical routing of wires. Some subscribers are near the central site while others are much farther away. Current practice has subscriber transceivers transmitting at close to maximum regulation power which is more than is generally required to maintain reliable communications with the central site at an optimum data rate, and contributes to cross-talk.
Attempts have been made at minimizing the cross-talk between the communications links described above. One proposal is for a better allocation of the frequency spectrum. Another is for implementation of power spectral density masks. Frequency coordination has been suggested as a way to control the use of the spectrum. The objective is to have different allocations for transmit and receive frequencies which prevent transmitters from occupying the receive spectrum. So far, however, there is no general agreement within the industry or regulatory bodies on frequency spectrum allocation. The power spectral density masks have been proposed but these limit the maximum transmit power in any frequency spectrum. So far there is no agreement in the industry as to the maximum transmit power permitted for many of the high frequency services currently being deployed.
There is a practical limit on the transmit signal power levels output by transceivers referred to as the xe2x80x9cmaximum transmit powerxe2x80x9d. There is also a practical minimum signal-to-noise ratio for enabling clear signal detection, referred to as a xe2x80x9clink marginxe2x80x9d. It is well known that signal attenuation occurs as a signal is carried by a twisted pair between transceivers and that attenuation increases with loop length. The quality of the loop and other components in the signal path between the subscriber transceiver and the provider transceiver also contribute to signal attenuation. Careful management of the physical facilities increases the loop reach and the reliability of communications, but it has been recognized that transmit signal power level control is required to enable a mass deployment of digital subscriber data services using a telephone network.
The current state of the art in subscriber line transmit signal power level control is described in Applicant""s co-pending U.S. patent application Ser. No. 09/031,647 to Darveau filed on Feb. 27, 1998, the entire specification of which is incorporated herein by reference. Darveau teaches that the amount of cross-talk within a data transmission system in which digital data is transmitted at high speeds over a telephone network can be reduced. Darveau also teaches the use of intelligence at the remote subscriber units to reduce differences in received data signal strength at a central site, thus reducing the potential for cross-talk corruption of the data. Cross-talk is reduced by analyzing a service provider""s signal received at a subscriber transceiver. All of the service provider""s tranceivers transmit at a known power level. By analyzing the strength of the signal received at the subscriber tranceiver, the subscriber transceiver is enabled to determine an appropriate transmit signal power level for its transmitted signal to achieve a target signal strength at the provider tranceiver. The subscriber""s transceiver transmit signal power is adjusted in coarse increments so that a data signal transmitted from the subscriber transceiver at the adjusted transmit signal power arrives at the central location at approximately the target signal strength.
Although this method has merit, it does not provide an optimal solution for reducing cross-talk between wire loops used for the delivery of high frequency data services.
As the volume and number of data services provided over twisted pair wire loops increases, transmission power management which results in more predictable and consistent data delivery rates and increased local loop reach is required.
It is therefore an object of the present invention to provide a method of transmission power management which accommodates a plurality of simultaneous communications sessions at a central site.
It is another object of the invention to provide a method of transmission power management which provides a more reliable data delivery.
It is another object of the invention to provide a method of transmission power management which enables a longer subscriber loop reach.
It is another object of the invention to provide a method of transmission power management which reduces the use of transmission power.
It is another object of the invention to provide a method of transmission power management which continuously optimizes transmission power usage.
It is yet another object of the invention to provide a method of transmission power management which optimizes transmit signal power levels based on existing connection conditions.
It is a further object of the invention to provide an apparatus for providing a data service over a twisted copper wire loop that enables a more reliable link, longer loop reach and higher transmit rates with lower bit error rates.
In accordance with the invention, there is provided a system for providing power control data delivery consistency between a central site equipped with provider transceivers and a plurality of remote subscriber transceivers located varying distances from the central site, each subscriber transceiver being connected to a provider transceiver by a wire loop, comprising:
a transmitter at each end of the wire loop, the transmitter including a transmit signal amplification stage having a command controllable transmit power attenuator;
a receiver at each end of the wire loop, the receiver including means for analyzing properties of a received signal; and,
at least one transmit signal power controller for each transceiver pair connected by a wire loop, the transmit signal power controller being adapted to receive the properties of the received signal and to control transmit power attenuation by the command controllable transmit power attenuator of a transmitter sending the received signal.
In accordance with a further aspect of the invention, there is provided a method of providing power control data delivery consistency between a central site equipped with provider transceivers and a plurality of remote subscriber transceivers, the provider transceivers and the subscriber transceivers being interconnected in pairs by a plurality of wire loops having various respective lengths, comprising the steps of:
receiving a communication signal at one of the transceivers transmitted by the other of the transceivers;
determining at least one predefined characteristic of the communication signal received;
determining from the at least one predefined characteristic whether the power level used to transmit the communication signal should be adjusted; and
issuing control commands to a command controllable transit power attenuator in the other of the transceivers to adjust the power level of the communication signal transmitted, if it is determined that the power level of the communication signal should be adjusted.
The invention therefore provides a method and apparatus for controlling transmit signal power levels in subscriber loops used for high frequency data services. Transmit signal power controllers in at least one of the provider and subscriber transceivers analyze communications signals transmitted by the other of the transceivers. The transmit signal power controllers examine a plurality of predetermined characteristics of the communication signal to ascertain whether the transmit signal power level should be adjusted. The goal is to regulate transmit signal power levels so that a minimum power for acceptable communications is dynamically maintained. Command controllable transit power attenuators are used to regulate transmit signal power levels. The command controllable transit power attenuators are preferably enabled to control transmit power outputs in small increments of 1-2 db.
Preferably, a transmit signal power controller and a command controllable transmit power attenuator are provided on each of the subscriber and provider transceivers. Proper control of transmit signal power levels reduces cross-talk within and between twisted pair binder groups, enables subscriber loop lengths to be extended and increases data transfer rates.